US2642322A

US2642322A - Roller bearing

Info

Publication number: US2642322A
Application number: US102406A
Authority: US
Inventors: Palmgren Nils Arvid
Original assignee: SKF AB
Current assignee: SKF AB
Priority date: 1948-07-21
Filing date: 1949-06-30
Publication date: 1953-06-16
Anticipated expiration: 1970-06-16
Also published as: DE825923C

Description

June 16, 1953 N. A. PALMGREN I 2,642,322

ROLLER BEARING v Filed June 30, 1949 Patented June 16,.1953

to Aktiebolaget Svenska Kullagerfabriken,

Goteborg, Sweden, a corporation of Sweden Application June 30, 1949, Serial No. 102,406

In Sweden July 21, 1948 -Y The problem of designing a roller bearing to obtain maximum load carrying capacity has been complicated by the lack of a theoretical basis for-the calculation of the stresses in the material which determine the strength of the bearing.

It has however now been found that there is a relation betweentheload and the life of the bearing to fatigue and that this life varies greatly, depending upon unavoidable variations in the strengthA of the material from point to point. The` result of this condition is, as has now been shown, that the strength of every part of the bearing influences the load carrying capacity and life of the bearing as a whole. The material stresses in every load carrying partof the bearing must therefore be as small as is possible for the load acting upon the bearing during running.

An earlier accepted conception that certain material stresses, e. g. the maximum normal pressure at the contact areas of the rollers, should be the same at both of the roller contacts is consequently incorrect. The 'material stress, which determines thefatigue, should instead be as small as possible in each ofthe contacts. As a rule this minimum does not have the same value at both contacts.

'The critical material stresses ina roller bearing occurin and adjacent' to the contact surl faces between the rollers and the races, and their magnitude depends not only on the load but also upon the shape of the areas of contact. s A`The present invention relates to a roller bearing in which the rollers and races are s formed that the contact areas will be elliptical for loads less than a certain value, i. e. that there is a certain, even if slight, difference in all directions between the curvatures of the contacting surfaces. The invention is characterized mainly thereby that the major axes of the elliptical contact areas between the roller and the races when the :roller is subjected to load have the same length up to a certain limit. I

Thereason why this arrangement solves the problem'of giving bearings of the type referred to a maximum of load carrying capacity is as follows. Since,when the curvature of the contacting surfaces is unchanged, the major axis ofthe vcontact ellipse depends solely on a function of the load andv the load on the roller from one of the races is always in. constant'relationzclaims. (C1. sos-212) ing drawing in which:

ship to the load von the roller from the other race, the major axes of the areas of contact of 2 constant relationship to each other. 1f the major axes are equal at one load, they will always be equal independent of the load. Because of the limited length of the roller however, a complete ellipse will be formed only at loads at which the length of the major axis is not greater than the length of the roller. When the load is greater, the contact will assume the form of a truncated ellipse and at a certain greater load, this figure will be transformed into a rectangle or a iigure resembling a rectangle. load, the material stresses will be of the same magnitude at the ends of the roller asat its middle, and the stresses in the contact are the smallest possible for the load in question. In a contact of any other form,` but with the same length, the same load lwould cause greater stresses. If the load is increased above this optimum, the material stresses at the ends of the roller increase at an unproportionate rate and edge pressure arises. This is a well-known phenomenon, which considerably lowers the carrying capacity and life of the bearing. f

At the optimal limit mentioned, the length of the major axis of the complete contact Vellipse is constant relative to the limited length vof the roller, usually inthe proportion 1.5 to 1, independent of the combination of principal curvatures VVin the contact.l

In order that the'load capacity of the bearing may be as great as possible, it is necessary that the optimum conditions occur at the same time in both contacts, which means that the bearing should be designed with relative curvatures of the rollers and races such that the major axes of the contact ellipses will be of the same length. The curvatures should also preferably be chosen so that the optimal value occurs at the maximum load to which the heaviest loaded roller in Ythe bearing is subjected duringuse.

The invention is illustrated Fig. 1 shows a cross section'through a double row spherical roller bearing; f

Fig. 2 showsfthe size of the' contact ellipses at the outer and inner races respectively fora small load; Y y Fig. 3 shows the contact ellipses at the 4opti-mal load; and

Fig. 4 shows the contact surfaces when the load is increased above this limit.

In Fig. 1 the numeral I indicates the outer ring of the bearing, which has a sphericalV race with the radiusv r2. The inner lring 2 is provided with two vraces havingconcave arcuate At this .limit y in the accompanygeneratrices, both with the radius r3. Two rows of rollers 3 and 4 are disposed between the race rings. The rollers have convexly curved generatrices, the curvature of which is r1. Hence, it will be apparent that the concavely curved generatrix of the bearing surface of the outer race I exhibits a flatter curvature, i. e. a curvature which more closely approaches a straight line, than the convexly curved generatrices of the bearing surfaces of the rollers 3 and A, and that the concavely curved generatrix of the bearing surface of the inner race 2 exhibits a flatter curvature than that of the said generatrix of the bearing surface of the outer race i. The rollers 3 and A are separated from one another and are guided by

cages

5 and 5, one for each row of rollers. A guide ring 1 is located between the rows of rollers 3 and 4 for assisting in guiding the rollers. The difference in the curvatures of the rollers and the sphere and races, respectively, is somewhat exaggerated in Fig. 1 of the accompanying drawing, in order to make the figure clearer.

The radius r1 of the generatrix of the roller is less than the radiusrz of the spherical surface and this in its turn is somewhat less than the radius r3 of the generatrix of the inner ring race. To state this relationship in other terms, it may be said that the radius of transverse curvature of the roller-engaging surface of the :1:

inner race 2 is greater than the radius of transverse curvature of the roller-engaging surface of the outer race l and is also greater than the radius of transverse curvature of each of the rollers 3 ande, i. e. the .transverse curvature of the roller-engaging surface of the cuter race has a value which lies between that of the rollerengaging surface of the inner race and the transverse curvature of each of the rollers 3 and 4. The major axes of the contact ellipses at the contact between a roller and the respective rings will therefore have the same length. This can be proved mathematically most easily through a calculation which shows that the major axis of the contact ellipse will be too great and consequently edge pressure will occur in the actual contact at -the inner race unless the radius of curvature of the generatrix of the inner raceway is greater than the radius of the sphere. An

example of a bearing according to the. invention 99.5 mm. A calculation of the major axis of the contact ellipse according to Hertz theory gives a value of 45.6 mm. for bearing parts of steel. This length is thus one and a half times the effective lengthv of the roller and the material stresses will therefore be the least possible for the load in question.

If the inner raceway is made with a generatrix having a radius of 100 mm. (i. e, equal tothe radius of the sphere), the length of the contact at this race would be 46.2 mm., i. e. 1.52 times the effective length of the roller, which means that the edge pressure would be greater than the pressure at the middle of the roller and that this contact would not meet the re quirements for minimum stresses. In order to lessen the calculated length of the contact to 1.5 times the effective length of the roller and thus obtain the optimum conditions, the length of the radius of the generatrix of the inner raceway must be increased somewhat so that it will be vgreater than the radius of the outer spherical raceway.

A check of the calculations of bearings of all current dimensions and proportions gives the same result and the rule is therefore general;

Fig. 2 shows the contact ellipses of a roller with the outer race i and inner race 2. The length of the major axis of the ellipse is in both cases S1. Fig. 2 relates to the case in which the load is comparatively small andthe length Si ofthe ellipse is therefore less than the length (l) of the roller. The contact ellipse at the outer race is wider than the ellipse at the inner vrace since the concave outer race curves toward the roller while the convex inner race curves n fail rst.

away from the roller in the direction of rolling.

Fig. 3 illustrates the optimal condition in which the areas of contact are substantially rectangular and the material stresses are the same at the ends of the roller as at its middle. If the length of the roller were unlimited, an elliptical contact having the length S2 would have been caused under this load. The length Sz would have been greater than the length of the roller in the proportion of 1.5 to 1.

Fig. 4 shows the character of the contact areas when the load exceeds the optimal limit. The contact widens out considerably at the ends of the rollers, since the material stresses at the edges of the roller are abnormally great.

In all of the cases shown, the pressures normal to the surfaces and other stresses in the contact areas are greater at the inner races than at the outer races. For the same number of repeated stresses on the same contact areas, fatigue would therefore probably occur rst at the inner race, but, because of the variation of the life, the outer race would also in some cases Since the outer race contact thus also affects the probable life of the bearing, this contact should be as strong as possible, and since it can be made stronger than the inner race contact, for the reasons given above, this possibility should be taken advantage of in order that the life and load carrying capacity of the bearing may be as great as possible.

The invention is not limited to bearings of the double row self-aligning type as described, but may be applied to all types of roller bearings which have point contacts at both races at low loads.

I claim:

1. A roller bearing having an outer race ring having a concavely curved bearing surface, an inner race ring having a concavely curved bearing surface, and a series of rolling members interposed vbetween said rings and each having a convexly curved bearing surface, the contacting surfaces of the said races and rollers differing from each other in curvature in all planes, the 'concavely curved generatrix of the bearing surface of the outer race ring exhibiting a flatter curvature than the convexly curved generatrices of the bearing surfaces of said rollers, and the concavely curved generatrix of the bearing surface ofthe inner race ring exhibiting a atter curvature than that of the generatrix of the bearing surface of said outer race ring, and the major axes of the areas of contact of an individual of said rollers with the bearing surfaces of the race rings when the bearing is under load being of the same lengthl at both the inner and outer race rings.

2. A roller bearing according to claim 1 wherein said bearing surface of said outer race ring is spherical, wherein said convexly curved bearing surface of each roller is on an arcuate generatrix of less radius than that of said spherical bearing surface on said outer race ring, and

wherein said bearing surface on said inner race ring is on an arcuate generatrx, the radius of the arcuate generatrix of said bearing surface on said inner race ring being greater than that of said spherical bearing surface on said outer race ring.

NILS ARVID PALMGREN.

References cited in the me of this Vpatent Fi Number 0 l 1,967,650 2,008,336

-UNITED STATES PATENTS vName n Date `Ahrnansson July 24, 1934 Palmgren July 16,1935 Gibbons June 1, 1937 Spicacci Apr. '1, 1947